There is a substantial risk of infection occurring in patients undergoing cancer chemotherapy. These infections arise either from the hospital environment, or from the patient's endogenous microbial flora. The protected environment-prophylactic antibiotic program significantly reduces the microbial burden and permits more intensive chemotherapy. Since infections still occur in these patients, more effective prophylactic measures must be developed. Many of the infections occurring in these patients are difficult to diagnose clinically. Routine monitoring of the microbial flora and of serological titers in patients undergoing chemotherapy should indicate potential for infection. Special culture techniques will permit the identification of bacterial L-forms, mycoplasma and anaerobes, when present. New antibiotic agents should improve the treatment of bacterial, fungal, viral and protozoal infections. Hence, the major objective is to develop improved methods for the diagnosis, treatment and prophylaxis of infection in cancer patients.